The present invention relates to the field of electro-optical displays, and more particularly to an optical imaging system in which a coherent optical signal is alternately directed through multiple optical modulation channels to increase the throughput rate of the system by avoiding delays caused by the time responses of the separate modulation channels.
Three dimensional (3D) volumetric displays project images in a true three-dimensional volume. This allows observers to view the image from any angle and has obvious advantages in terms of depth and contour perception. A 3D display operates by projecting voxels, which constitute each point in the 3D image. The voxels are projected into a three dimensional volume. A voxel is the 3D analogue of a pixel, the latter representing a point of a two dimensional image.
Projected 3D displays can be effectively produced using a visible laser illumination source in conjunction with a laser scanner. Each voxel is located at a specific x, y, and z coordinates in space. The x,y coordinates determine the location of the voxel in the plane that is generally perpendicular to the beam propagation axis, while the z coordinate represents location of the voxel along the laser beam propagation axis. A beam deflector directs the laser beam to the x,y location of the voxel. The z or third coordinate of the voxel must also be established in order to produce the 3D image. This can be accomplished by temporarily locating a screen or other suitable scattering surface at the x,y, and z coordinates of the voxel.
An effective method for establishing the z coordinate of each voxel is to use a rotating helical surface. As the helix rotates about an axis that is oriented parallel to the laser propagation axis it provides a projection surface that varies along the z axis in a repetitive, regular manner. At any given time in the rotational cycle the points on the helical surface can be identified with specific x, y, and z coordinates. And in the course of one complete rotational cycle the helical surface will pass through all x, y, and z coordinates contained within the scanning volume. The dimensions of the scanning volume are defined by the overall dimensions of the rotating helix.
If a steady laser beam is projected towards the rotating helical surface at fixed x,y coordinates, it appears to the observer to be moving back and forth along the z axis. When x,y deflection is provided for the laser beam 3D images may be created since the projection screen moves through all x, y, and z coordinates in the scanning volume during each cycle. The 3D image is created voxel by voxel. As long as all voxels are generated within the time determined by human retinal persistence (about {fraction (1/20)} of a second) the image appears continuously and produces no flicker.
There are two types of x,y laser scanning techniques that have been used in conjunction with a spinning helix to produce 3D volumetric displays. The first is raster scanning, where all x and y coordinates are addressed in a sequential fashion, generally line by line. For x,y pairs that are not needed to produce an image voxel the laser beam is blanked, or turned off. The second scanning technique is called random access scanning. This technique directs the laser beam to only those x,y pairs that are required to produce the image voxels. Each x,y coordinate is loaded into the scanning apparatus as required to form the image. Random access scanning is a more efficient manner of creating a 3D image.
Modem volumetric displays utilize acousto-optic deflectors to direct the laser beam to the x,y coordinates of each voxel. An acousto-optic deflector contains a transparent optical crystal through which the beam passes. An acoustic wave is generated in the crystal in the plane of the propagation axis of the laser. By creating periodic variations in the refractive index of the crystal the acoustic wave generates a transmission-type diffraction grating through which the beam must propagate. The frequency of the standing acoustic wave determines the period of the grating, which in turn determines the angular deflection of the laser beam.
Establishing a grating within the acousto-optic crystal takes some time. The grating dimension transverse to the laser propagation direction must be at least as large as the laser beam diameter. In addition, each time new x,y coordinates are desired a new grating with a different period must be established within the crystal. The time required to produce the grating with the proper dimensions in the crystal is termed the access time or fill time of the crystal. During the access time the laser beam is blanked to avoid unwanted artifacts in the image. This reduces the optical throughput efficiency of the scanner and prevents efficient image projection. For the extreme case where the access time is equal to the voxel xe2x80x9con time the scanner efficiency is zero.
Another problem limiting the efficiency of scanners used in current 3D volumetric displays is related to the manner in which the laser light is utilized. Typically for high speed volumetric displays more than one acousto-optic bean deflector is utilized. This is done to increase the total number of voxels in a 3D projected image. Each x,y beam deflector is located in a separate xe2x80x9cchannel. The input laser beam must be divided among the various channels reducing the maximum instantaneous laser intensity available for each voxel to the total laser power divided by the number of channels. However, the minimum amount of laser power per voxel is determined by the power required to produce a visible voxel, that is, a voxel that may be observed. Therefore the effect of dividing the input laser power into two or more channels reduces the visibility of each voxel for a given laser input power and voxel on-time.
Thus, it may be appreciated that there is a need for a high speed random access laser scanner that provides accurate 3D volumetric display images in a manner that makes efficient use of the total input laser power and improves the visibility of the image voxels.
A 3D volumetric display is produced by using a laser in conjunction with acousto-optic scanning and a rotating helical projection surface. Random access scanning is used to deflect the laser beam to the x,y coordinates of each image voxel. The illuminating laser beam is switched sequentially among the several acousto-optic deflecting channels. Switching among the channels can be accomplished by using a Pockels cell device. Alternatively switching may be produced by using a moving mirror or a rotating disk. The timing of the blanking, switching and deflecting signals is coordinated to produce high speed, high resolution, efficient 3D images.
The invention may also be characterized as an optical switch for selectively directing an optical signal along one or another of two axes. The optical switch includes a Pockels cell for transforming a first polarization state of the optical signal into a second polarization state in response to receiving an input signal; and a birefringent mirror which allows the optical signal to propagate along a first axis when the optical signal has a first polarization state, or for directing the optical signal along a second axis when the optical signal has a second polarization state.
The invention further includes a system for directing optical energy through a selected one of multiple optical modulation channels. The system includes an optical switch having first and second optical output ports for selectively directing an optical signal out of one of either a first or second optical output port; a first optical modulation channel for modulating the output signal received from the first optical output port; and a second optical modulation channel for modulating the output signal received from the second output port.
The invention may also be characterized as a method for directing an optical signal along a selected one or another of two different axes. The method includes the steps of: generating an optical signal having a first polarization state; propagating the optical signal along a first axis when the optical signal has said first polarization state; transforming the first polarization state into a second polarization state when an input signal has a first value; passing the optical signal along the first axis when the light signal has the first polarization state; or directing the optical signal along the second axis when the light signal has the second polarization state.
An important advantage of the invention is that the projected image brightness is improved. Another advantage is that efficient operation is achieved. A further advantage of the invention is that the total laser input power may be used to illuminate each image voxel. Yet another advantage of the invention is the avoidance of the time delay engendered by the acousto-optic deflector crystal fill-time. Still another advantage of the invention is that a higher data throughput rate is achieved. These and other advantages of the invention will become more readily apparent from the ensuing Specification and drawings when taken in conjunction with the appended claims.